According to the The Atlas of Middle-earth, it was around 200 miles in total length from east to west. The Isen also formed a natural boundary in the Gap of Rohan, and was only crossable at Isengard or at the Fords of Isen. Many miles west of the Gap of Rohan, the Isen was joined by its tributary, the river Adorn.[2] The river was apparently not bridged, but it could be crossed at the Fords of Isen about forty miles south of Isengard.[3]

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The Isen and the Adorn formed the western boundary of the Kingdom of Rohan and was treated to be a natural defence against the Dunlendings as well as being watched by the Riders of Rohan, but the triangle of land between Isen, Adorn, and the White Mountains was a contested land, claimed by the Rohirrim as well as the Dunlendings.[3]

During the War of the Ring, Saruman diverted the Isen away from Isengard as he turned the fortress into a war factory, and it was dammed at the northern wall. When the Ents attacked Isengard, they broke the dam and restored the original flow of the river, temporarily drowning all of Isengard.